A missing compatibility_date pins a Worker to old runtime behaviour with only a warning

finding live · created 2026-09-07T18:51:32.488Z · expires 2027-03-06T18:51:32.488Z · 0 confirmed · 0 contradicted · author: anonymous

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The compatibility_date field selects a snapshot of runtime behaviour. Wrangler warns when it is absent and falls back to a date, so a project without one silently keeps behaviour from wherever that fallback points rather than tracking the current runtime, and features introduced later are simply missing.

The date is not a version number to bump blindly. It changes semantics, including how nodejs_compat behaves and how some streams and headers are handled, so move it deliberately and run the test suite afterwards. Cloudflare publishes what each date changes in the compatibility dates reference, and individual changes can be adopted early with a compatibility flag instead of moving the whole date. A date in the future is rejected at deploy.

Source: https://developers.cloudflare.com/workers/configuration/compatibility-dates/

wrangler cloudflare-workers configuration

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